The present invention relates to a projection type aligner, especially to an alignment detection optical system suited for a reduction projection aligner used for the manufacture of the micropattern LSIs.
The manufacturing processes of the LSI need to include a process for accurately superimposing and printing the pattern of another circuit on the pattern of the circuit which has already been formed on a substrate such as a wafer of the semiconductor. The device for printing such circuit pattern is the exposure system. The pattern to be printed is formed on a mask, and the action for aligning a wafer pattern to a mask pattern is called the alignment. In order to meet the demand for higher integration and further reduction of the LSI pattern, the reduction projection aligner is used, and the TTL (Through The Lens) alignment system has come to be used for the alignment between the alignment pattern of the mask and the alignment pattern image of the wafer and the subsequent exposure.
As the alignment detection optical system for observing the alignment pattern of the mask and the alignment pattern image of the wafer at the time of the above-mentioned TTL alignment, there is already an optical system harmless to the circuit pattern to be exposed, adaptable to the TTL alignment and adjustable to the variation of the chip size as is announced in Japanese Patent Application Laid-Open No. 142612/1982.
According to said optical system, the alignment pattern image of the wafer is projected on an image sensor through a conventional projection optical system, so that the pattern signal is dependent on the characteristic of the reduction lens. In designing the reduction lens, however, the first priority is given to its resolution for obtaining the desired exposure and printing effects, therefore the numerical aperture (NA) of the lens is relatively large. It is a characteristic of the lens that the image of the alignment pattern which is formed as a step on the surface of the wafer and projected on the mask has a weak contrast. Thus, the alignment pattern signal obtained by said optical system has a weak contrast, the drawback to the improvement in the alignment.